[meteorite-list] grains of sand

From: Robin Whittle <rw_at_meteoritecentral.com>
Date: Fri, 08 Nov 2013 13:34:40 +1100
Message-ID: <527C4DC0.3000606_at_firstpr.com.au>

Hi Bob,

You wrote, in part:

> I also feel that the too-often-used phrase "nothing made it to the
> ground - it all burned-up" is too ill-informed. How is it physically
> possible for a cobble-pebble-sandgrain to continue traveling fast
> enough to completely ablate down to total nothingness? It's my
> opinion that (depending on altitude) for all sizes of incoming debris
> (even at cosmic-velocity) there is a certain retardation-point where,
> once it is reached, ablation can no longer occur. I would not be
> surprised if this minimum size is in the fine-pebble grain-size
> range, which is certainly still "findable". One way that this
> conjecture could be substantiated, is if there were actually a gap in
> the population of small meteorites between pebble and micro-spherule.
> Has this already been recorded in Antarctica?

It never made sense to me that every part of shattered meteoroid would
vaporise. I think there would be two requirements for this to be the case:

1 - Each fragment would need to be too small to survive the ablation or
    further fragmentation (with each fragment being subject to this
    condition recursively) which prevents a final solid fragment slowing
    and cooling so it falls to the ground in a size and location where
    it can be found and identified as part of the fall.

2 - Both ablation and fragmentation depend on velocity and density of
    air, and the smaller the fragment the faster it loses the velocity
    required for ablation and further fragmentation.

As far as I know, most meteorites are found via visual search, though in
some cases metal detectors play a role.

While I recall that there have been some studies of meteorite dust by
collecting material which falls on a roof or other special collecting
system, these experiments are not typically - or ever - at the site of a
recent meteorite fall.

Perhaps if the dust particles such as 2mm and smaller, were preserved
and highly visible, such as on a white sand or salt-lake surface, they
might be found. I haven't heard of anyone in the Chelyabinsk area
searching for dust in the run-off from their roofs, which were generally
covered in snow at the time.

Dust particles would have been clearly visible on the top of the snow
immediately after the fall, but I don't know of any reports of these.
If the meteoroid really was 10,000 tonnes, I would have thought there
would be lots of dust particles falling to the ground.

The only explanation I can think of which would account for dust sized
particles being evaporated entirely would be that the entire environment
of high altitude air they are in was raised to the required temperature
primarily by compression, and a little by friction. Still, I can't
imagine how all or even most fragments could be vaporised.

These dust particles could be blown far away and so be difficult to find
since they would be highly dispersed. There were strong winds in the
Chelyabinsk event. This would preferentially drop sand-grain sized
remnants further downwind, so it would not be surprising if there was
little or no "dust" in the main strewn field.

Still, theoretically, one might expect a full distribution of meteorite
sizes from the main mass down to smoke particles, with wind-driven
sorting of these by size to fall different distances from Lake
Cherbarkul. With the snow-bound area, wide publicity and huge volume of
material, I think the Chelyabinsk event should be a good opportunity to
research the fate of smaller particles.

On eBay, there are fragments down to fractions of a gram. I guess it
would be hard to sell grain of sand like particles, and difficult to
prove they were actually part of the fall, unless by destructive testing.

Surely the "smoke" cloud of the Chelyabinsk meteor contained some dust
which would have fallen. Even micron-sized smoke particles surely
settle to Earth under gravitational forces, given enough time either
directly by falling or more likely by being caught in a cloud and then a
raindrop and falling as rain, hail or snow. If so, then in principle
filtering some air here in Australia might, in principle, find a few
tiny particles of the Chelyabinsk meteoroid.

All the above is based on limited knowledge, how I imagine things might
work out in the upper atmosphere and how this doesn't fit well with what
I know about the observations. No doubt folks on this list can do
better than I can with references, observations and theories.

   - Robin
Received on Thu 07 Nov 2013 09:34:40 PM PST